The FGD of a coal, oil or lignite power station separates SO2 produced in the combustion process, and prevents SO3, that is to say, sulfuric acid, from being formed in the atmosphere from this SO2. This is because this sulfuric acid rains down and acidifies soils (acid rain) and has been partly responsible for the death of forests in many areas of Europe.
The chemical process in FGD binds SO2 with limestone or other reagents to form gypsum. In this case, many different chemical reactions occur, in which a whole series of substances may participate, which are typically an integral part of the flue gas or have been introduced into the process together with the FGD reagent, usually limestone. These include fly ash, chlorides, fluorides, minerals, etc.
Experience gained from the FGD process in a large power station shows that caked-on deposits are repeatedly formed on the walls and on the internal components (pipelines, nozzles, beams, etc.) of FGD. These caked-on deposits are partly the result of the oxidation process of the SO2 and partly the result of chemical processes of the other constituents of the flue gas and the limestone. These deposits often arise from chemical crystallization processes and are therefore very hard and rigid. They often have the hardness and consistency of clay or porcelain fragments and frequently also their appearance.
These caked-on deposits may easily come loose from the substrate during operation. This loosening is caused by temperature fluctuations (thermal expansion and contraction break up the deposits and loosen them) or mechanical forces, such as vibration or pressure pulses. These caked-on deposits then fall into the scrubber sump. They have various sizes with a diameter of 10 mm to 1000 mm.
These caked-on fragments and solids can have a considerable harmful effect in the FGD process. They may damage FGD components, they may lead to accelerated wear, or they may disrupt the process considerably due to blockages.
The following are examples of typical damage and disruptions:
1. Damage to the Pump                In the case of rubber-lined recirculation pumps, these hard caked-on fragments, sometimes even having sharp edges, may damage the rubber lining and consequently cause corrosion on the pump body.        
2. Abrasion on the Recirculation Lines                The inner layer of the recirculation line, whether rubber lining, GFRP or polypropylene, is ground and worn down by these large and hard bodies when the suspension liquid flows upward to the spraying level.        
3. Blockage of Spraying Levels and Spray Nozzles                The large pieces collect at the spraying levels to form larger heaps. They flow more slowly than the suspension liquid. When a sufficient number of large parts comes together, this may easily lead to a situation where the nozzle pipe is blocked at a narrower point or where the parts in the spray nozzle come to a stop and block it.        
4. Abrasion at the Spraying Levels                These caked-on fragments are thrown by the spray nozzles onto the spraying levels lying underneath and cause abrasion and damage when they impinge onto the surface.        
5. Gypsum Dewatering                In the gypsum dewatering process, these parts may likewise cause disruptions and problems.        
It is therefore in the interests of power stations to keep the larger of these caked-on fragments in the FGD sump and to avoid a situation where these fragments infiltrate into other structural parts and cause damage or disruption there.
This is the object of suspension solution filter sieve baskets which are installed upstream of the pumps and upstream of the outlets for gypsum dewatering. These suspension solution filter sieve baskets keep the large solid parts in the FGD sump and avoid the situation where they penetrate into the other process steps and there cause the damage described.
These suspension solution filter sieve baskets are conventionally manufactured with holes of 20 mm to 25 mm so as to retain all parts which have an equal or larger diameter.
These sieves were first manufactured from high-grade steels. This, on the one hand, had the effect that the procurement and installation of these suspension solution filter sieve baskets became a highly cost-intensive investment. On the other hand, it was found that high-grade steel is suitable for only a very limited extent for this caking-on.
Severe corrosion of these suspension solution filter sieve baskets therefore repeatedly occurred, and the suspension solution filter sieve baskets had to be exchanged. Increasingly higher-alloyed steels had to be used in order to avoid this corrosion (which naturally made the costs even higher).
On the other hand, it was found that caked-on deposits were also formed on the steel. The suspension solution filter sieve baskets regularly grew in size over time. The pressure loss rose and the sieves had to be cleaned offline.
Sieves made from GFRP were also used. Here it was also found that the function was restricted. On the one hand, damage to the GFRP repeatedly occurred, which then led to GFRP corrosion (loosening of the glass fibers from the resin), and on the other hand breakage damage repeatedly occurred due to large caked-on deposits falling down and the rapid pressure changes when the pumps were switched on and off. It became apparent that GFRP was a material unsuitable for this application.
Thermoplastics, such as polypropylene, proved to be the most suitable for this application. On the one hand, thermoplastics are corrosion-free even in this environment, and on the other hand, the suspension solution filter sieve baskets made from thermoplastic proved to be robust and unsusceptible to contamination. There was neither damage, as in the case of GFRP, nor did massive caked-on deposits occur, as in the case of high-grade steel. The majority of all new FGD has since been equipped with suspension solution filter sieve baskets made from polypropylene.
At the temperatures prevailing in the FGD sump, polypropylene is relatively soft and has only limited stability. The suspension solution filter sieve basket therefore must be produced from a relatively thick material (20 mm polypropylene board) and additionally has to be stiffened with ribs.
It nevertheless happens repeatedly that suspension solution filter sieve baskets are damaged during operation. This damage is usually caused by the fact that caked-on fragments are trapped in the holes of the suspension solution filter sieve basket and remain stuck in. The pipe is then largely or completely blocked. When a large number of holes is blocked, the pressure loss rises, and increasingly the pump suction force is led to the body of the suspension solution filter sieve basket. This then at some point leads to the deformation of the suspension solution filter sieve basket, to the point of its destruction.
Power stations naturally keep an eye on the operation of the suspension solution filter sieve baskets and try to recognize imminent damage and prevent this in advance. The current consumption of the pumps or the pressure loss of the pump is measured. When current consumption rises or the pressure loss conditions change, this indicates that a large proportion of the holes in the suspension solution filter sieve basket are blocked.
Internal online washing systems were experimented with. It became apparent, however, that these cleaning mechanisms can be used to only a very restricted extent underwater. Another method was therefore adopted: these suspension solution filter sieve baskets are flushed free by “backflush”. The pump is stopped and the liquid in the recirculation lines is not discharged via the dewatering outflow, but instead is allowed to flow back into the sump again via the suspension solution filter sieve basket. This backflow through the suspension solution filter sieve basket frees the suspension solution filter sieve basket of caked-on parts which are stuck in the holes of the suspension solution filter sieve basket or even only lie against the holes and are held there by the suction force. A flap or sieve basket cover in the suspension solution filter sieve basket prevents the falling pressure of the up to 25 m high liquid column in the recirculation line from throwing the suspension solution filter sieve basket out of its fastening.
It was also found, however, that this backflush helps to only a limited extent. Only part of the caked-on deposits stuck in the holes is removed. Another part of the caked-on deposits which increases in size over time remains stuck in the holes of the suspension solution filter sieve basket and must be removed manually during inspection.
An analysis of such caked-on fragments stuck firmly in suspension solution filter sieve baskets showed that this has something to do with the configuration of the holes or with the manufacturing process. These holes have hitherto been drilled out by means of drills. Cylindrical holes were therefore obtained.
An exact analysis of the manufacturing technique shows, however, that this manufacturing method has a considerable disadvantage with regard to the problem of contamination. To be precise, after the drilling of the holes, the board has to be bent into a semicircle which then forms the typical shape of the suspension solution filter sieve basket. The semicircle is important so as to give the relatively soft material the static strength to withstand the forces of the pump without deformation in the event of contamination.
It is known, then, that shaping a board into a semicircle causes deformation of the material. The outer side of the board is expanded and the inner side of the semicircle is compressed. This means, for the conical hole obtained by drilling, that it has then assumed a conical shape. The diameter on the outside of the suspension solution filter sieve basket has been widened and the diameter on the inside of the suspension solution filter sieve basket has been compressed, that is to say reduced. Moreover, the hole is no longer round.
This shape of the hole is disadvantageous for the intended use. This conical shape of the hole means, then, that solids can penetrate into the hole with the liquid stream because they are somewhat smaller than the outer inlet, but they can no longer leave the hole inwardly because the solid is too large.
This situation is made even worse in that the surface in the hole has become rough and irregular due to production (drilling or milling). This irregularity has even been intensified by bending.
It may be that this problem seems minor. It appears somewhat unlikely at first sight that the solids always have exactly the size of the hole.
Three factors must, however, be taken into account:
1. 8,000-16,000 Hours of Operation                The power station should ideally operate for 8,000 to 16,000 hours without disruption and interruption, and many thousand cubic meters are drawn through the suspension solution filter sieve basket every hour. Over this long period of time, it is not unexpected that a relatively large number of solids appear in exactly the correct size.        
2. Agitator Mechanisms                The sump is permanently kept in motion via agitator mechanisms in order to prevent suspended substances from settling and solidifying and in order to improve the oxidation of the SO2. New solids are therefore constantly carried to the suspension solution filter sieve baskets.        
3. Growth and Abrasion                During this constant motion, the shape and size of the solids also change. On the one hand, the size is reduced by abrasion and collision and, on the other hand, the solids constantly grow in size on account of the oxidation processes.        
The sieves become increasingly blocked over time. By the “backflush” described above, blockage can be repeatedly reduced, but to a lesser extent over time. Blockages therefore occur which cannot be eliminated by means of the “backflush”.
These are usually solids stuck in the holes which have not been able to be washed out of the suspension solution filter sieve basket by the final “backflush” (emptying of the recirculation lines during rundown).
The problem is aggravated in that the solids which are stuck in the holes begin to grow as a result of chemical crystallization processes. The slow growth observed in the metallic suspension solution filter sieve baskets occurs in an only very minor form in the case of the thermoplastic suspension solution filter sieve baskets. However, a growth of the holes may nevertheless occur when a solid is stuck firmly in the hole and grows in size.